Use and production of nanotubes containing a mixed valence venadium

ABSTRACT

Separable nanotubes are made from a transition metal oxide, preferably from a vanadium oxide of variable valence. They show a greater oxidation resistance than the carbon-based nanotubes known so far and offer many new and economic applications. The inventive nanotubes clearly show oxidation-reduction activities and are particularly suited as an active material for catalytic reactions.

BACKGROUND OF THE INVENTION

Nanotubes and the process for their production is known according to thecurrent state-of-the-art. The nanotubes are needle-shaped tubes with alength of 1,000 Å, for example, and a diameter of a few 100 Å. Nanotubesare separated or separable and do not all point in one specificdirection. Known are also sclerogenous structures, which are created bya network of tubes. The tubes of such a sponge-like structure have aninside diameter that is growth-limited to be not larger than 80 Å.

The currently known nanotubes and also the above-mentioned sclerogenousstructures consist mainly of carbon and are produced by arcing.Reference is made to this in JP-A-071 65 406, JP-A-071 97 325 and inscientific discussions published in NATURE, volume 358, pages 220through 222 and volume 363, pages 603 through 605. Known are alsonanotubes made of gold or titanium oxide, as it can be seen in apublication of Langmuir, volume 12, number 6, 1996, pages 1411 through1413.

Single nanotubes may be used, for example, to catch individualmolecules. Nanotubes may be handled individually or they may be pastedon for microscopic examinations. However, carbon nanotubes havespecifically the disadvantage that they are unstable relative tooxidizing effects.

SUMMARY OF THE INVENTION

The innovative nanotubes are characterized in that they are made of atransition metallic oxide. Such nanotubes may also be produced to beseparable, whereby they are essentially not aligned in athree-dimensional manner. They are considerably more oxidation stablethan carbon nanotubes and have additional advantageous characteristicsand applications, which cannot be found in carbon nanotubes. Inparticular, the innovative nanotubes have distinct redox activities,which offer a number of novel uses and applications. It has been shownthat nanotubes made of mixed-valence vanadium oxide are especiallyadvantageous. They are especially oxidation stable and may be producedin form of a solution.

Innovative nanotubes and specifically nanotubes made from mixed-valencevanadium oxide are well suited as active agents for catalytic reactions.Special advantageous applications are the ones for the process of redoxreaction in batteries as storage electrodes or in high capacitycapacitors. Additonal advantageous uses of the innovative nanotubes maybe seen in the process for exchange reactions of the tube content, whichmay include molecules, metallic clusters, metalloid clusters orpolymers, and specifically so-called molecular wires. The nanotubes actin a size-selective manner so that proteins of a certain size may beplaced into the tubes.

The invention, according to the minor claims, relates also to anadvantageous process for the production and preferred use of nanotubes.

BRIEF DESCRIPTION OF THE DRAWINGS

Application examples of the invention are explained in more detailbelow. The enclosed illustrations show the following:

FIG. 1 shows and electron-microscopic partial view of a nanotube.

FIG. 2 shows an electron-microscopic view of a group of nanotubes.

FIG. 3a shows a scanning electron-microscopic view of open vanadiumoxide nanotubes with a length of up to 2,000 nm.

FIG. 3b shows an EM picture of low definition, which also shows thatisolated or intergrown vanadium oxide nanotubes are created as the mainproduct after hydrothermal analysis.

FIGS. 3c and 3 d show a high-definition TEM picture, whereby a space ofapproximately 3 nm between layers may be clearly observed according tothe multi-layer characteristic of the oxidized walls. The free diameterof the illustrated tubes are between approximately 25 and 35 nm and theouter measurements are between 50 and 70 nm.

FIG. 3e shows an X-ray diffraction diagram of a powder test of aninnovative vanadium oxide nanotube, whereby the reflex of highestintensity has a value of approximately 3.3 nm, which corresponds to thelayer space in the TEM picture.

FIG. 3f shows a picture of nanotubes with closed ends, whereby the ratioof alkoxide to template (Templat) is 1:1.

FIG. 4 shows a high-definition TEM picture of a cation-exchangednanotube, where the layer contrasts show only a space of 0.86 nmaccording to the arrow markings and where some areas show a finestructure, which suggests a systematic structure in the individual walllayers and between the layers.

DETAILED DESCRIPTION OF THE INVENTION

The innovative nanotubes are arranged in multi-layered tube formationsas shown in the electron-microscopic picture, whereby the individualtubes are open at both ends—but they could also be closed. Theindividual tubes are separable as already mentioned. The nanotubes aremade of mixed-valence vanadium in their preferred version. They appearin form of black powder. The invention also includes nanotubes made ofother transition metallic oxides, specifically vanadium oxide,molybdenum oxide, zinc oxide and indium oxide.

An example for the production of the innovative nanotubes made ofvanadium oxide or molybdenum oxide is shown below. Vanadium is replacedby molybdenum in case of molybdenum oxide.

Vanadium oxide-triisopropoxide is added to 1.89 g (7.87 mmol) ofhexadecylamine under argon atmosphere of 3.8 g (15.74 mmol) and it isthen agitated for one hour by adding 5 ml of absolute ethanol. Thecreated solution was later hydrolyzed, whereby orange precipitationoccurred, which was aged during agitation for one day. This reactionmixture was then poured into an autoclave (43 ml volume) and wassubsequently heated for one day to 80° C., the next day to 100° C. andfor seven days to 180° C. The black reaction product was separated byfiltering after cooling and was washed with 50 ml water, 50 ml ethanoland 20 ml ethyl ether and was subsequently dried in air.

Analytics:

[C] 38.47 weight percent [H] 7.72 weight percent [N] 2.72 weight percent

Hexadecylamine may be substituted with another neutral tenside molecule.Vanadium oxide-triisopropoxide may also be substituted with anothermetallic alkoxide, specifically with molybdenum oxide. The quantityratio of the tenside molecule is of importance. Suitable ratio forquantity of substance is 1:2 and 1:3.

The hydrolysis of vanadium (V)—triisopropoxide in hexadecylaminesolution resulted in a yellow lamellar structured composite made of asurfactant and vanadium (V)-oxide. The even layer spaces in thismaterial are approximately 3 nm. Subsequent treatment under hydrothermalcondition create a product of isolated or star-shaped intergrownnanotubes with the empirical formula VO₂₄(C₁₆H₃₃NH₂)_(0.34).

The black powdery material is paramagnetic and has semi-metallicelectronic conduction characteristics, which is based on themixed-valence vanadium centers. The electron-microscopic pictures showalmost exclusively isolate or star-shaped intergrown nanotubes (FIG. 3a)having a length of up to 2,000 nm. The cylindrical open ends of thetubes are easily seen through the scanning electron microscope (FIG.3b). Highly defined TEM pictures (FIG. 3c and 3 d) show that the tubeshave diameters between 15 and 100 nm and tunnel openings between 5 and50 nm. In addition, the walls consist of concentric shells with athickness of approximately 3 nm. That corresponds exactly to thestrongest X-ray reflex in the powder diagram (FIG. 3e). This product isidentified here with ETH1, following the common practice for zeoliticand mesoporous materials. Tubes with closed ends may also be obtainedunder slightly changed synthesis conditions (see FIG. 3f).

Fascinating is the observation that the spaces between the layers in thetube walls become smaller after detemplatization (ETH2, FIG. 4) fromETH1 (approximately 2.8 to 3.0 nm) to approximately 0.86 nm (FIG. 4,arrow markings). A cooperative process must be assumed since most tubesremain in existence during this reaction. It is interesting that thecrystallinity of the exchanged product is considerably better than theone of ETH1. In some cases, smaller contrast spaces of approximately0.65 nm may be observed on the tube surfaces, which indicate acrystalline order within the wall layers and in between the layers (FIG.4, arrow markings). This may also be interpreted as the considerablyhigher thermal capacity of the tubes in the electron beam. The exchangedand structural altered material is here identified as ETH2.

In contrast to the much smaller carbon nanotubes, the VO₂-tubes areobtained as the main product of a direct chemical synthesis at relativelow temperatures. Compared to other oxidic mesoporous substances, likeMCM41 or MCM49, there are no condensing ring systems in this case andthe tunnel diameters are greater up to a factor of 10. Besides, nomulti-layered walls were observed in MCM-materials, which could be areason that intermediate created and isolated aluminosilicate tubes havenot been detected up to now. The innovative nanotubes offer fascinatingpossibilities for further chemical and physical exploration of nano-sizestructures and they could be a fruitful stimulant for potentialapplications. Especially the general cooperative cation exchange betweenthe layers offers a number of experiments in regard of an insertion offunctional cations or cation systems. Even though the size of the tubesmay be of interest in relation to a large material surface, we believethat the tubes may be used above all for the absorption of largefunctional molecules and for the combination of inorganic materials, toinclude polymeric organic or even biological substances. It is alsoimportant that the multi-layered material of the nanotubes may also havetemplate molecules in the intermediate layers, which may be removed by asimple cation exchange whereby the tubes may still be retained.

What is claimed is:
 1. Nanotubes comprising a mixed valence vanadiumoxide containing both vanadium IV oxide and vanadium V oxide. 2.Nanotubes according to claim 1, wherein said nanotubes further compriseunloaded template (Templat) molecules.
 3. Nanotubes according to claim1, wherein the nanotubes further comprise contain an amine.
 4. Nanotubesaccording to claim 1, wherein the nanotubes are formed in the shape of ashell.
 5. Nanotubes according to claim 1, wherein the inside diameter issmaller than 500 Å.
 6. Nanotubes according to claim 1, wherein theoutside diameter is smaller than 1,100 Å.
 7. Nanotubes according toclaim 3, wherein the amine is hexadecylamine.